Method and device for fixing a ceramic component in a metallic support

ABSTRACT

The invention relates to a method for the fixing of a ceramic component ( 4 ) in a metallic support ( 2 ). The method is characterized in that, in a first method step, a bush ( 5 ), consisting of gray cast iron with lamellar graphite, is placed around the ceramic component ( 4 ), in a second method step, the ceramic component ( 4 ) including the bush ( 5 ) are annealed at temperatures in the range from 500 to 750° C. until the bush ( 5 ) has an increase in volume as a result of internal oxidation that causes the ceramic component ( 4 ) to be permanently fixed and in that finally, after cooling down, the ceramic component ( 4 ) with the bush ( 5 ) are introduced into the metallic support ( 2 ) and secured in the support ( 2 ).

[0001] The invention relates to a method and a device for fixing aceramic component in a metallic support. A ceramic component of thistype is, for example, the ceramic insulating bush for the high-voltageignition conductor which is arranged in the ignition torch of a gasturbine.

[0002] It is known prior art to use ceramic adhesives to cementinsulating bushes made of Al₂O₃ in the metallic support of the ignitiontorch of a gas turbine. The insulating bushes insulate the high-voltageignition conductor from the metallic support.

[0003] To ensure a good connection between the metallic support and theinsulating bushes, heat curing of the adhesive must take place.

[0004] This known type of fastening of ceramic insulating bushes in ametallic support has a series of disadvantages. On the one hand, theceramic adhesive is very difficult to introduce into the intermediatespace between the insulating bush and the metallic support. Sometimes itmust be injected into the intermediate space with a syringe, which isdifficult to handle. The ceramic adhesive used requires a very narrowgap. This has unfavorable effects on assembly. On the other hand, theadhesive shrinks considerably during curing (by about 20 to 30%), whichleads to a reduction in the adhesive, with the result that the bush isoften cemented only poorly in the metallic support and the adherence isinadequate.

[0005] The invention is based on the object of eliminating thesedisadvantages of the known prior art and developing a method for fixinga ceramic component in a metallic support and a device for this purposewhich is easy to handle, always ensures good adherence between theceramic component and the metallic support and is availableinexpensively and everywhere.

[0006] According to the invention, this is achieved in the case of amethod for the fixing of a ceramic component in a metallic support by abush consisting of gray cast iron with lamellar graphite being placedaround the ceramic component in a first method step, the ceramiccomponent including the bush being annealed at temperatures in the rangefrom 500 to 750° C. in a second method step, until the bush has anincrease in volume as a result of internal oxidation that causes theceramic component to be permanently fixed by shrinkage and, aftercooling down, the ceramic component with the bush being introduced intothe metallic support and secured in the support.

[0007] According to the invention, this is achieved in the case of adevice for the fixing of a ceramic component in a metallic support bythe device comprising a bush which is produced from gray cast iron withlamellar graphite and is arranged at those locations of the ceramiccomponent at which the ceramic component is to be fixed in the metallicsupport, the ceramic component being fixed in the bush by shrinkage onaccount of internal oxidation of the gray cast iron with lamellargraphite. The ceramic component is, for example, an insulating bush fora high-voltage ignition conductor which is arranged in the ignitiontorch of a gas turbine.

[0008] The advantages of the invention are that a permanent connectioncan be produced by a simple annealing operation, without specialassembly know-how being necessary. Since the increase in volume of thegray cast iron occurs only where oxygen enters during the annealingoperation, uniform surface pressure occurs between the ceramic componentand the bush made of gray cast iron. A self-healing effectadvantageously occurs, because at temperatures above 500° C. theoxidation recommences where the connection has loosened. It stops whereno more oxygen enters. In this way, very good adherence is achieved.Finally, it is also an advantage of the invention that the connectioncan be nondestructively released again at high temperature (about 700 to800° C.) by pressing out.

[0009] It is expedient if the annealing of the ceramic componentincluding the bush made of gray cast iron with lamellar graphite iscarried out at a temperature of approximately 700° C. At thistemperature, the internal oxidation takes place in a comparatively shorttime, which has favorable effects on production costs.

[0010] It is also advantageous if the bush made of gray cast iron withlamellar graphite is secured in the metallic support by center punchmarks on the support. This is a simple and quick method.

[0011] It is also of advantage if GG 20-25 is used as the material forthe bush, because this material is available everywhere and iscomparatively inexpensive. What is more, the graphite flakes are opentoward the outside, so that in this way oxygen is brought into thematerial, making the internal oxidation and the associated increase involume possible in the first place.

[0012] Finally, it is expedient if glass ceramic with a similar modulusof elasticity and coefficient of thermal expansion as the bush made ofgray cast iron with lamellar graphite is used as the material for theceramic component, because in this way no additional longitudinalstresses are produced in the ceramic in the event of temperaturechanges.

[0013] An exemplary embodiment of the invention is represented in thedrawing, in which:

[0014]FIG. 1 shows a partial longitudinal section of the ignition torchof a gas turbine;

[0015]FIG. 2 shows a cross section through the glass ceramic and graycast-iron bush after the annealing treatment;

[0016]FIG. 3 shows a diagram in which the dependence of the pressingforce caused by internal oxidation of GG 20 on the annealing durationand annealing temperature is represented.

[0017] Only the elements essential for the invention are represented anddesignated.

[0018] The invention is explained in more detail below on the basis ofan exemplary embodiment and FIGS. 1 to 3.

[0019] In FIG. 1, a partial longitudinal section of the ignition torch 1of a gas turbine is represented. A high-voltage ignition conductor 3 isarranged in a metallic support 2, for example made of St S 18/8. Thishigh-voltage ignition conductor 3 is insulated from the metallic support2 by means of a plurality of ceramic components 4, which are arrangedaround the circumference of the high-voltage ignition conductor 3,spaced apart from one another in the axial direction of the high-voltageignition conductor 3. In FIG. 1, two ceramic components 4 are depictedby way of example. In this exemplary embodiment, the components 4 areinsulating bushes made of glass ceramic. Glass ceramic can be machinedwell by turning and has a similar modulus of elasticity and coefficientof thermal expansion as gray cast iron. Arranged around the components 4at the locations at which they are to be fixed in the metallic support 2are bushes 5 made of gray cast iron with lamellar graphite, here GG 20.The ceramic components 4 are anchored in the bushes 5 on account ofinternal oxidation of the gray cast iron with lamellar graphite, i.e.the ceramic is fixed in terms of pressure.

[0020] The method according to the invention is characterized in thatthe bush 5, consisting of gray cast iron with lamellar graphite,preferably GG 20-25 (DIN 1691), is placed around the ceramic component 4in a first method step, the ceramic component 4 including the bush 5,i.e. as one part, are annealed at temperatures in the range from 500 to750° C. in a second method step, until the bush 5 has an increase involume as a result of internal oxidation that causes the ceramiccomponent 4 to be permanently fixed, and in that finally, after coolingdown, the ceramic component 4 with the bush 5 is introduced into themetallic support 2 and secured in the support 2 in the last method step.This securement preferably takes place simply by means of center punchmarks on the support 2. The placement points for the securing centerpunches (blind holes for placing the center punch when securing againstslipping out) are designated in FIG. 1 by 6.

[0021] To be specified as preferred treatment parameters for the jointannealing of the ceramic component 4 and the gray cast-iron bush 5 areapproximately 700 to 750° C. and 2 days (48 hours). At these treatmenttemperatures, the internal oxidation takes place in a comparativelyshort time, which has favorable effects on production costs.

[0022]FIG. 2 shows a cross section through the glass-ceramic and graycast-iron bush after the annealing treatment. The ceramic component 4 isfirmly connected to the bush 5 made of GG 20 and is consequently fixedin terms of pressure by frictional forces.

[0023] The advantages of the invention are that a permanent connectionof a ceramic component 4 and a bush 5 can be produced by a simpleannealing operation, without special assembly know-how being necessary.Since the increase in volume of the gray cast iron with lamellargraphite occurs only where oxygen enters during the annealing operation,uniform surface pressure occurs between the ceramic component 4 and thebush 5 made of gray cast iron. A self-healing effect advantageouslyoccurs, because at temperatures above 500° C. the oxidation recommenceswhere the connection has loosened. It stops where no more oxygen enters.In this way, very good adherence is achieved. Finally, it is also anadvantage of the invention that the connection can be nondestructivelyreleased again at high temperature by pressing out. A further advantageof the invention is that GG 20 to GG 25 can be used as the material forthe bush. This material is inexpensive and easily available everywhere.

[0024] What is more, the graphite flakes are open toward the outside, sothat in this way oxygen is brought into the material, making theinternal oxidation and the associated increase in volume possible in thefirst place. If gray cast iron with nodular graphite were used insteadof gray cast iron with lamellar graphite, the invention would not beeffective, because no oxygen uptake over a relatively large volume canoccur and consequently no internal oxidation can occur.

[0025]FIG. 3 shows in a diagram the dependence of the force which isnecessary to press the ceramic component 4 out of the bush 5 made of GG20 after oxidation in—as a function of the annealing duration andannealing temperature. The pressing force increases with increasingannealing duration (10-60 hours) and increasing annealing temperature(550-750° C.).

[0026] It goes without saying that the invention is not restricted tothe exemplary embodiment described.

List of designations

[0027]1 ignition torch

[0028]2 metallic support

[0029]3 high-voltage ignition conductor

[0030]4 ceramic component, for example insulating bush

[0031]5 bush made of gray cast iron with lamellar graphite

[0032]6 placement point for securing center punch

[0033] t annealing duration

[0034] T annealing temperature

[0035] F pressing force

1. A method for the fixing of a ceramic component (4) in a metallicsupport (2) characterized in that a) in a first method step, a bush (5),consisting of gray cast iron with lamellar graphite, is placed aroundthe ceramic component (4), b) in a second method step, the ceramiccomponent (4) including the bush (5) are annealed at temperatures in therange from 500 to 750° C. until the bush (5) has an increase in volumeas a result of internal oxidation causing the ceramic component (4) tobe permanently fixed and c) after cooling down, the ceramic component(4) with the bush (5) are introduced into the metallic support (2) andsecured in the support (2).
 2. The method as claimed in claim 1 ,characterized in that the annealing of the ceramic component (4)including the bush (5) made of gray cast iron with lamellar graphite iscarried out at a temperature of approximately 700 to 750° C.
 3. Themethod as claimed in claim 1 , characterized in that the bush (5) madeof gray cast iron with lamellar graphite is secured in the metallicsupport (2) by center punch marks on the support (2).
 4. The method asclaimed in claim 1 , characterized in that GG 20-25 is used as thematerial for the bush (5).
 5. The method as claimed in claim 1 ,characterized in that glass ceramic with a similar modulus of elasticityand coefficient of thermal expansion as the bush (5) made of gray castiron with lamellar graphite is used as the material for the ceramiccomponent (4).
 6. A device for the fixing of a ceramic component (4) ina metallic support (2), characterized in that the device comprises abush (5) which is produced from gray cast iron with lamellar graphiteand is arranged at those locations of the ceramic component (4) at whichthe ceramic component (4) is to be fixed in the metallic support (4),the ceramic component (4) being firmly anchored with the bush (5) onaccount of internal oxidation of the gray cast iron with lamellargraphite.
 7. The device as claimed in claim 6 , characterized in thatthe ceramic component (4) is an insulating bush for a high-voltageignition conductor (3).